Generally, trickle charging of an over-drained battery pack, e.g., a Lilon-type battery pack, can be used to avoid damaging the battery pack and to increase the battery life. FIG. 1 shows a conventional battery charging system 100 for controlling trickle charging of an over-drained battery pack.
In FIG. 1, the conventional battery charging system 100 can control trickle charging of an over-drained battery pack 114 via a charging switch 104, e.g., a charging field-effect transistor (FET), coupled to a current limiting resistor 102 in series. The charging FET 104 is coupled to a trickle charging control pin PCHG of a controller 116. A charging switch 106, e.g., a charging FET, coupled to a normal charging control pin CHG of the controller 116 is used to charge the battery pack 114 in a normal condition. A discharging switch 110, e.g., a discharging FET, coupled to a discharging control pin DSG of the controller 116 is used to discharge the battery pack 114. One of the drawbacks of the battery charging system 100 is that an extra charging switch, e.g., the charging FET 104, and an extra trickle charging control pin, e.g., the trickle charging control pin PCHG, are needed. As a result, the cost of the battery charging system 100 and the pin count of the controller 116 can be increased. Furthermore, if the controller 116 is powered by the battery pack 114, the controller 116 may not be operable when a voltage of the battery pack 114 is close to zero. Thus, it may be difficult to turn on the charging FET 104 when the voltage of the battery pack 114 is close to zero.